Resist composition and patterning process

ABSTRACT

A resist composition comprising a base resin comprising recurring units having an acid labile group and recurring units containing a rubidium or cesium salt of α-fluorinated sulfonic acid exhibits a high sensitivity and forms a pattern of satisfactory profile with minimal LWR after exposure and development.

CROSS-REFERENCE TO RELATED APPLICATION

This non-provisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No. 2015-208575 filed in Japan on Oct. 23, 2015, the entire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

This invention relates to a resist composition and a patterning process using the same.

BACKGROUND ART

To meet the demand for higher integration density and operating speed of LSIs, the effort to reduce the pattern rule is in rapid progress. The wide-spreading flash memory market and the demand for increased storage capacities drive forward the miniaturization technology. As the advanced miniaturization technology, manufacturing of microelectronic devices at the 65-nm node by the ArF lithography has been implemented in a mass scale. Manufacturing of 45-nm node devices by the next generation ArF immersion lithography is approaching to the verge of high-volume application. The candidates for the next generation 32-nm node include ultra-high NA lens immersion lithography using a liquid having a higher refractive index than water in combination with a high refractive index lens and a high refractive index resist film, EUV lithography of 13.5 nm wavelength, and double patterning version of the ArF lithography, on which active research efforts have been made.

The current technology is approaching to the processing size which is reduced below 50 nm as minimum line width. When the processing size is so reduced, the thickness of resist film must be reduced below 100 nm, depending on the surface material of the substrate to be processed, because of such factors as the structural strength to maintain the pattern against the surface tension of developer and the adhesion strength to the substrate. On use of prior art chemically amplified resist materials intended to form high-resolution resist film, for example, based on a base resin having an acetal protective group, no significant degradation of line edge roughness (LER) does occur with a resist film having a thickness of 150 nm, but LER is substantially exacerbated when the film thickness is reduced below 100 nm.

With respect to high-energy radiation of very short wavelength such as EB or x-ray, hydrocarbons used in resist materials have little absorption. Then hydrocarbon (typically polyhydroxystyrene) base resist materials are under consideration. Resist materials for EB lithography are practically used in the mask image writing application. Recently, the mask manufacturing technology becomes of greater interest. Reduction projection exposure systems or steppers have been used since the time when the exposure light was g-line. While their demagnification factor was ⅕, a factor of ¼ is now used as a result of chip size enlargement and projection lens diameter increase. It becomes of concern that a dimensional error of a mask has an impact on the dimensional variation of a pattern on wafer. It is pointed out that as the pattern feature is reduced, the value of a dimensional variation on the wafer becomes greater than the value of a dimensional error of the mask. This is evaluated by a mask error enhancement factor (MEEF) which is a dimensional variation on wafer divided by a dimensional error of mask. Patterns on the order of 45 nm often show an MEEF in excess of 4. In a situation including a demagnification factor of ¼ and a MEEF of 4, the mask manufacture needs an accuracy substantially equivalent to that for equi-magnification masks.

The exposure system for mask manufacturing made a transition from the laser beam exposure system to the EB exposure system to increase the accuracy of line width. Since a further size reduction becomes possible by increasing the accelerating voltage of the electron gun in the EB exposure system, the accelerating voltage increased from 10 kV to 30 kV and reached 50 kV in the current mainstream system, with a voltage of 100 kV being under investigation.

As the accelerating voltage increases, a lowering of sensitivity of resist film becomes of concern. As the accelerating voltage increases, the influence of forward scattering in a resist film becomes so reduced that the contrast of electron image writing energy is improved to ameliorate resolution and dimensional control whereas electrons can pass straightforward through the resist film so that the resist film lowers its sensitivity. Since the mask exposure tool is designed for exposure by direct continuous writing, a lowering of sensitivity of resist film leads to an undesirably reduced throughput. Due to a need for higher sensitivity, chemically amplified resist compositions are contemplated.

As the feature size is reduced, image blurs due to acid diffusion become a problem (see Non-Patent Document 1). To insure resolution for fine patterns with a size of 45 nm et seq., not only an improvement in dissolution contrast is requisite, but control of acid diffusion is also important (see Non-Patent Document 2). Since chemically amplified resist compositions are designed such that sensitivity and contrast are enhanced by acid diffusion, an attempt to minimize acid diffusion by reducing the temperature and/or time of post-exposure bake (PEB) fails, resulting in drastic reductions of sensitivity and contrast.

Addition of an acid generator capable of generating a bulky acid is effective for suppressing acid diffusion. It is then proposed to incorporate in a polymer recurring units derived from an onium salt having polymerizable olefin serving as acid generator. Sulfonium and iodonium salts having a polymerizable unsaturated bond capable of generating a sulfonic acid are proposed in Patent Document 1 and other documents. Patent Document 1 also discloses a sulfonium or iodonium salt having sulfonic acid directly attached to the backbone.

It was avoided to use metal-containing materials as the lithography resist material for the semiconductor device fabrication because of a possible malfunction of semiconductor devices. However, it is known in the application other than the semiconductor, for example, as the resist material for forming color filters for LCD, to use a metal-containing (meth)acrylate as a copolymerizable monomer.

Patent Document 2 discloses EB resist and antistatic film having alkali metal and alkaline earth metal salts added thereto. These salts improve the sensitivity on EB exposure at no sacrifice of resolution. Patent Document 3 discloses a chemically amplified resist composition having a metal salt of carboxylic acid or β-diketone added thereto. The metal salt of carboxylic acid or β-diketone functions as a quencher as it undergoes ion exchange with a sulfonic acid generated from the acid generator.

CITATION LIST

-   Patent Document 1: JP-A 2006-178317 -   Patent Document 2: JP-A 2010-152136 -   Patent Document 3: JP-A 2013-025211 (U.S. Pat. No. 9,360,753) -   Non-Patent Document 1: SPIE Vol. 5039 p1 (2003) -   Non-Patent Document 2: SPIE Vol. 6520 p65203L-1 (2007)

DISCLOSURE OF INVENTION

While the miniaturization of the pattern rule is in progress to meet the demand for higher integration density and operating speed of LSIs as alluded to previously, there is a need for a resist composition which has a high sensitivity despite a high resolution and forms a pattern of satisfactory profile and minimal roughness (LWR) after exposure and development.

An object of the invention is to provide a resist composition which has both high resolution and sensitivity and forms a pattern with a satisfactory profile and minimal LWR after exposure and development; and a patterning process using the same.

The inventors have found that a resist composition, typically chemically amplified positive resist composition, comprising a base resin comprising recurring units having an acid labile group and recurring units containing a rubidium or cesium salt of α-fluorinated sulfonic acid exhibits a high sensitivity, forms a pattern of satisfactory profile with minimal LER after exposure and development, and is effective for preventing electrostatic charges during EB image writing. The resist composition is thus suited as the micropatterning material for the fabrication of VLSIs and photomasks.

In one aspect, the invention provides a resist composition comprising a base resin comprising recurring units containing an acid labile group and recurring units having the formula (a).

Herein R¹ is hydrogen or methyl, X¹ is phenylene or —C(═O)—X³—R²—, X³ is —O— or —NH—, R² is a single bond, or a C₁-C₂₀ straight, branched or cyclic alkylene group, C₂-C₂₀ straight, branched or cyclic alkenylene group, C₂-C₂₀ straight, branched or cyclic alkynylene group, or C₆-C₁₀ aryl group, which may contain an ether, ester, lactone ring, carbonate, amide, carbamate, sulfonic acid ester or sultone moiety, X² is a single bond, a C₁-C₂₀ straight, branched or cyclic alkyl group, C₂-C₂₀ straight, branched or cyclic alkenyl group, C₂-C₂₀ straight, branched or cyclic alkynyl group, or C₆-C₁₀ aryl group, which may contain halogen, ether, thiol, ester, carbonate, carbonyl, amide, amino, azide, carbamate, nitro, cyano, hydroxyl, carboxyl, sulfo, sulfonic acid ester, sultone, lactone ring or lactam ring, and M⁺ is a rubidium or cesium ion.

In a preferred embodiment, the recurring units containing an acid labile group have the formula (b1) or (b2).

Herein R³ and R⁵ are each independently hydrogen or methyl, R⁴ and R⁶ each are an acid labile group, Y¹ is a single bond, a C₁-C₁₂ linking group having at least one of ester, lactone ring, phenylene and naphthylene moiety, a phenylene group, or a naphthylene group, Y² is a single bond, ester group or amide group, b1 and b2 are numbers in the range: 0≤b1≤0.9, 0≤b2≤0.9, and 0<b1+b2<1.

In a preferred embodiment, the base resin further comprises recurring units of at least one type selected from the formulae (c1), (c2) and (c3).

Herein R¹⁰¹, R¹⁰⁵, and R¹¹⁰ each are hydrogen or methyl, R¹⁰² is a single bond, phenylene, —O—R¹¹⁴—, or —C(═O)—Y—R¹¹⁴—, Y is —O— or —NH—, R¹¹⁴ is a C₁-C₆ straight, branched or cyclic alkylene group, C₃-C₁₀ alkenylene or phenylene group, which may contain a carbonyl, ester, ether or hydroxyl moiety, R¹⁰³, R¹⁰⁴, R¹⁰⁷, R¹⁰⁸, R¹⁰⁹, R¹¹¹, R¹¹², and R¹¹³ are each independently a C₁-C₁₂ straight, branched or cyclic alkyl group which may contain a carbonyl, ester or ether moiety, or a C₆-C₁₂ aryl, C₁-C₂₀ aralkyl, or mercaptophenyl group in which at least one hydrogen may be replaced by a C₁-C₁₀ straight, branched or cyclic alkyl, halogen, trifluoromethyl, cyano, nitro, hydroxyl, alkoxy, alkoxycarbonyl or acyloxy moiety, R¹⁰⁶ is hydrogen or trifluoromethyl, Z¹ is a single bond or —C(═O)—Z³—R¹¹⁵—, Z² is a single bond, methylene, ethylene, phenylene, fluorinated phenylene, —O—R¹¹⁵— or —C(═O)—Z³—R¹¹⁵, Z³ is —O— or —NH—, R¹¹⁵ is a C₁-C₆ straight, branched or cyclic alkylene group, phenylene group, or C₁-C₆ straight, branched or cyclic alkenylene group, which may contain a carbonyl, ester, ether or hydroxyl moiety, a pair of R¹⁰³ and R¹⁰⁴, R¹⁰⁷ and R¹⁰⁸, R¹⁰⁷ and R¹⁰⁹, R¹⁰⁸ and R¹⁰⁹, R¹¹¹ and R¹¹², R¹¹¹ and R¹¹³, or R¹¹² and R¹¹³ may bond directly or via a methylene moiety or ether bond to form a ring with the sulfur atom to which they are attached, Q⁻ is a non-nucleophilic counter ion, c1, c2 and c3 are numbers in the range of 0≤c1≤0.5, 0≤c2≤0.5, 0≤c3≤0.5, and 0<c1+c2+c3≤0.5. Of these units, recurring units having formula (c2) are preferred.

Typically the resist composition is a chemically amplified positive resist composition.

The resist composition may further comprise at least one component selected from among an organic solvent, a dissolution inhibitor, an acid generator, a basic compound, and a surfactant.

In another aspect, the invention provides a pattern forming process comprising the steps of coating the resist composition defined above onto a substrate, baking, exposing the resulting resist film to high-energy radiation, and developing with a developer. The high-energy radiation is typically EUV of wavelength 3 to 15 nm or EB at an accelerating voltage of 1 to 150 kV.

In the step of exposing the resist film to high-energy radiation, preferably the surface of the substrate underneath the resist film is electrically charged positive.

ADVANTAGEOUS EFFECTS OF INVENTION

The resist composition has many advantages including a significantly high contrast of alkaline dissolution rate before and after exposure, a high sensitivity, a high resolution, exposure latitude, process adaptability, a satisfactory pattern profile after exposure, a controlled rate of acid diffusion, and a minimal LWR. The resist composition, typically chemically amplified positive resist composition is suited as the micropatterning material for VLSIs and photomasks, and the patterning material in the EB and EUV lithography. The resist composition is used not only in the lithography for semiconductor circuit formation, but also in the formation of mask circuit patterns, micro-machines, and thin-film magnetic head circuits.

DESCRIPTION OF EMBODIMENTS

The singular forms “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise. “Optional” or “optionally” means that the subsequently described event or circumstance may or may not occur, and that the description includes instances where the event occurs and instances where it does not. As used herein, the notation (C_(n)-C_(m)) means a group containing from n to m carbon atoms per group. Me stands for methyl and Ac for acetyl.

The abbreviations and acronyms have the following meaning.

UV: ultraviolet radiation

DUV: deep ultraviolet

EUV: extreme ultraviolet

EB: electron beam

Mw: weight average molecular weight

Mn: number average molecular weight

Mw/Mn: molecular weight distribution or dispersity

GPC: gel permeation chromatography

PEB: post-exposure bake

LER: line edge roughness

LWR: line width roughness

Resist Composition

The invention provides a resist composition comprising a base resin comprising recurring units containing a rubidium or cesium salt of sulfonic acid which is fluorinated at α-position and recurring units having an acid labile group.

The recurring unit containing a rubidium or cesium salt of α-fluorinated sulfonic acid is represented by the following formula (a), the recurring unit being simply referred to as unit (a).

Herein R¹ is hydrogen or methyl. X¹ is phenylene or —C(═O)—X³—R²—, wherein X³ is —O— or —NH—, and R² is a single bond, or a C₁-C₂₀ straight, branched or cyclic alkylene group, C₂-C₂₀ straight, branched or cyclic alkenylene group, C₂-C₂₀ straight, branched or cyclic alkynylene group, or C₆-C₁₀ aryl group, which may contain an ether, ester, lactone ring, carbonate, amide, carbamate, sulfonic acid ester or sultone moiety. X² is a single bond, a C₁-C₂₀ straight, branched or cyclic alkyl group, C₂-C₂₀ straight, branched or cyclic alkenyl group, C₂-C₂₀ straight, branched or cyclic alkynyl group, or C₆-C₁₀ aryl group, which may contain halogen, ether, thiol, ester, carbonate, carbonyl, amide, amino, azide, carbamate, nitro, cyano, hydroxyl, carboxyl, sulfo, sulfonic acid ester, sultone, lactone ring or lactam ring. M⁺ is a rubidium (Rb) or cesium (Cs) ion.

Upon exposure, the inventive resist composition relies on the mechanism that acid is generated by an acid generator which is separately added or included in recurring units (c1) to (c3) to be described later, rather than the mechanism that the bond is cleaved to generate acid. Upon exposure, the metal in the recurring unit (a) emits secondary electrons, which cause to decompose the acid generator, leading to an improvement in sensitivity.

The monomer from which the recurring unit (a) is derived has the formula (a′).

Herein R¹, X¹, X² and M⁺ are as defined above.

Examples of the monomer having formula (a′) are given below, but not limited thereto. R¹ and M⁺ are as defined above.

The monomer having formula (a′) may be synthesized, for example, by neutralization reaction or ion exchange reaction of an α-fluorinated sulfonic acid having a polymerizable group or an ammonium salt of an α-fluorinated sulfonic acid having a polymerizable group with rubidium hydroxide, cesium hydroxide, rubidium carbonate, cesium carbonate, rubidium β-diketone complex, or cesium β-diketone complex.

The recurring units containing an acid labile group are preferably units having the formula (b1) or units having the formula (b2), which are simply referred to as units (b1) or (b2).

Herein R³ and R⁵ are each independently hydrogen or methyl. R⁴ and R⁶ each are an acid labile group. Y¹ is a single bond, a C₁-C₁₂ linking group having at least one of ester, lactone ring, phenylene and naphthylene moiety, a phenylene group, or a naphthylene group. Y² is a single bond, ester group or amide group, b1 and b2 are numbers in the range: 0≤b1≤0.9, 0≤b2≤0.9, and 0<b1+b2<1.

Examples of the monomer from which the recurring unit (b1) is derived are shown below, but not limited thereto. R³ and R⁴ are as defined above.

Examples of the monomer from which the recurring unit (b2) is derived are shown below, but not limited thereto. R⁵ and R⁶ are as defined above.

The acid labile groups represented by R⁴ and R⁶ may be selected from a variety of such groups. The acid labile groups may be the same or different and preferably include groups of the following formulae (A-1) to (A-3).

In formula (A-1), R^(L1) is a tertiary alkyl group of 4 to 20 carbon atoms, preferably 4 to 15 carbon atoms, a trialkylsilyl group in which each alkyl moiety has 1 to 6 carbon atoms, an oxoalkyl group of 4 to 20 carbon atoms, or a group of formula (A-3). A1 is an integer of 0 to 6.

Exemplary tertiary alkyl groups are t-butyl, t-pentyl, 1,1-diethylpropyl, 1-ethylcyclopentyl, 1-butylcyclopentyl, 1-ethylcyclohexyl, 1-butylcyclohexyl, 1-ethyl-2-cyclopentenyl, 1-ethyl-2-cyclohexenyl, and 2-methyl-2-adamantyl. Exemplary trialkylsilyl groups are trimethylsilyl, triethylsilyl, and dimethyl-t-butylsilyl. Exemplary oxoalkyl groups are 3-oxocyclohexyl, 4-methyl-2-oxooxan-4-yl, and 5-methyl-2-oxooxolan-5-yl.

Examples of the acid labile groups of formula (A-1) include t-butoxycarbonyl, t-butoxycarbonylmethyl, t-pentyloxycarbonyl, t-pentyloxycarbonylmethyl, 1,1-diethylpropyloxycarbonyl, 1,1-diethylpropyloxycarbonylmethyl, 1-ethylcyclopentyloxycarbonyl, 1-ethylcyclopentyloxycarbonylmethyl, 1-ethyl-2-cyclopentenyloxycarbonyl, 1-ethyl-2-cyclopentenyloxycarbonylmethyl, 1-ethoxyethoxycarbonylmethyl, 2-tetrahydropyranyloxycarbonylmethyl, and 2-tetrahydrofuranyloxycarbonylmethyl groups.

Also included are groups having the formulae (A-1)-1 to (A-1)-10.

Herein R^(L8) is each independently a straight, branched or cyclic C₁-C₁₀ alkyl group or C₆-C₂₀ aryl group, R^(L9) is hydrogen or a straight, branched or cyclic C₁-C₁₀ alkyl group, R^(L10) is each independently a straight, branched or cyclic C₂-C₁₀ alkyl group or C₆-C₂₀ aryl group, and A1 is as defined above.

In formula (A-2), R^(L2) and R^(L3) each are hydrogen or a straight, branched or cyclic alkyl group of 1 to 18 carbon atoms, preferably 1 to 10 carbon atoms. Exemplary alkyl groups include methyl, ethyl, propyl, isopropyl, n-butyl, s-butyl, t-butyl, cyclopentyl, cyclohexyl, 2-ethylhexyl, and n-octyl. R^(L4) is a monovalent hydrocarbon group of 1 to 18 carbon atoms, preferably 1 to 10 carbon atoms, which may contain a heteroatom such as oxygen, examples of which include straight, branched or cyclic alkyl groups and substituted forms of such alkyl groups in which some hydrogen atoms are replaced by hydroxyl, alkoxy, oxo, amino, alkylamino or the like. Illustrative examples of the substituted alkyl groups are shown below.

A pair of R^(L2) and R^(L3), R^(L2) and R^(L4), or R^(L3) and R^(L4) may bond together to form a ring with the carbon atom or the carbon and oxygen atoms to which they are attached. A ring-forming combination of R^(L2) and R^(L3), R^(L2) and R^(L4), or R^(L3) and R^(L4) is a straight or branched alkylene group of 1 to 18 carbon atoms, preferably 1 to 10 carbon atoms, while the ring preferably has 3 to 10 carbon atoms, more preferably 4 to 10 carbon atoms.

Of the acid labile groups of formula (A-2), the straight and branched ones are exemplified by the following groups having formulae (A-2)-1 to (A-2)-69.

Of the acid labile groups of formula (A-2), the cyclic ones are, for example, tetrahydrofuran-2-yl, 2-methyltetrahydrofuran-2-yl, tetrahydropyran-2-yl, and 2-methyltetrahydropyran-2-yl.

Other examples of acid labile groups include those of the following formula (A-2a) or (A-2b) while the polymer may be crosslinked within the molecule or between molecules with these acid labile groups.

Herein R^(L1) and R^(L12) each are hydrogen or a straight, branched or cyclic C₁-C₈ alkyl group, or R^(L11) and R^(L12), taken together, may form a ring with the carbon atom to which they are attached, and a ring-forming combination of R^(L11) and R^(L12) is a straight or branched C₁-C₈ alkylene group. R^(L13) is independently a straight, branched or cyclic C₁-C₁₀ alkylene group. Each of B1 and D1 is an integer of 0 to 10, preferably 0 to 5, and C1 is an integer of 1 to 7, preferably 1 to 3.

“A” is a (C1+1)-valent aliphatic or alicyclic saturated hydrocarbon group, aromatic hydrocarbon group or heterocyclic group having 1 to 50 carbon atoms, which may be separated by a heteroatom or in which one or more carbon-bonded hydrogen atoms may be substituted by hydroxyl, carboxyl, carbonyl moieties or fluorine atoms. Preferably, “A” is a straight, branched or cyclic C₁-C₂₀ alkylene, alkyltriyl or alkyltetrayl group, or C₆-C₃₀ arylene group. “B” is —CO—O—, —NHCO—O— or —NHCONH—.

The crosslinking acetal groups of formulae (A-2a) and (A-2b) are exemplified by the following formulae (A-2)-70 through (A-2)-77.

In formula (A-3), R^(L5), R^(L6) and R^(L7) each are a monovalent hydrocarbon group, typically a straight, branched or cyclic C₁-C₂₀ alkyl group or straight, branched or cyclic C₂-C₂₀ alkenyl group, which may contain a heteroatom such as oxygen, sulfur, nitrogen or fluorine. A pair of R^(L5) and R^(L6), R^(L5) and R^(L7), or R^(L6) and R^(L7) may bond together to form a C₃-C₂₀ aliphatic ring with the carbon atom to which they are attached.

Exemplary tertiary alkyl groups of formula (A-3) include t-butyl, triethylcarbyl, 1-ethylnorbornyl, 1-methylcyclohexyl, 1-ethylcyclopentyl, 2-(2-methyl)adamantyl, 2-(2-ethyl)adamantyl, and t-pentyl.

Other exemplary tertiary alkyl groups include those of the following formulae (A-3)-1 to (A-3)-18.

Herein R^(L14) is each independently a straight, branched or cyclic C₁-C₈ alkyl group or C₆-C₂₀ aryl group, typically phenyl, R^(L15) and R^(L17) each are hydrogen or a straight, branched or cyclic C₁-C₂₀ alkyl group, and R^(L16) is a C₆-C₂₀ aryl group, typically phenyl.

Also useful are acid labile groups having the following formulae (A-3)-19 and (A-3)-20. The polymer may be crosslinked within the molecule or between molecules with these acid labile groups.

Herein R^(L14) is as defined above, R^(L18) is a (E1+1)-valent, straight, branched or cyclic C₁-C₂₀ aliphatic hydrocarbon group or C₆-C₂₀ aromatic hydrocarbon group, which may contain a heteroatom such as oxygen, sulfur or nitrogen, and E1 is an integer of 1 to 3.

Examples of the recurring units having an acid labile group of formula (A-3) include recurring units of (meth)acrylate having an exo-form structure represented by the formula (A-3)-21.

Herein R³ is as defined above. R^(Lc1) is a straight, branched or cyclic C₁-C₈ alkyl group or an optionally substituted C₆-C₂₀ aryl group. R^(Lc2) to R^(Lc7), R^(Lc10) and R^(Lc11) are each independently hydrogen or a monovalent C₁-C₁₅ hydrocarbon group which may contain a heteroatom. R^(Lc8) and R^(Lc9) are hydrogen. Alternatively, a pair of R^(Lc2) and R^(Lc3), R^(Lc4) and R^(Lc6), R^(Lc4) and R^(Lc7), R^(Lc5) and R^(Lc7), R^(Lc5) and R^(Lc11), R^(Lc6) and R^(Lc10), R^(Lc8) and R^(Lc9), or R^(Lc9) and R^(Lc10), taken together, may form a ring with the carbon atom to which they are attached, and a ring-forming combination is a divalent C₁-C₁₅ hydrocarbon group which may contain a heteroatom. Also, a pair of R^(Lc2) and R^(Lc11), R^(Lc8) and R^(Lc11), or R^(Lc4) and R^(Lc6) which are attached to vicinal carbon atoms may bond together directly to form a double bond. The formula also represents an enantiomer.

The monomers from which recurring units having formula (A-3)-21 are derived are exemplified in U.S. Pat. No. 6,448,420 (JP-A 2000-327633). Illustrative non-limiting examples of suitable monomers are given below. R³ is as defined above.

The recurring units having an acid labile group of formula (A-3) include those units derived from (meth)acrylates having furandiyl, tetrahydrofurandiyl or oxanorbornanediyl, represented by the formula (A-3)-22.

Herein R³ is as defined above. R^(Lc12) and R^(Lc13) are each independently a monovalent, straight, branched or cyclic C₁-C₁₀ hydrocarbon group, or R^(Lc12) and R^(Lc13), taken together, may form an aliphatic ring with the carbon atom to which they are attached. R^(Lc14) is furandiyl, tetrahydrofurandiyl or oxanorbornanediyl. R^(Lc15) is hydrogen or a monovalent, straight, branched or cyclic C₁-C₁₀ hydrocarbon group which may contain a heteroatom.

Examples of the monomers from which the recurring units having formula (A-3)-22 are derived are shown below, but not limited thereto.

Monomers containing an acid labile group other than the foregoing are exemplified in U.S. Pat. No. 9,335,633 (JP-A 2015-166833, paragraphs [0061]-[0085]).

The base resin may further comprise recurring units of at least one type selected from the formulae (c1), (c2) and (c3), which are simply referred to as units (c1), (c2) and (c3).

Herein R¹⁰¹, R¹⁰⁵ and R¹¹⁰ each are hydrogen or methyl. R¹⁰² is a single bond, phenylene, —O—R¹¹⁴—, or —C(═O)—Y—R¹¹⁴, wherein Y is —O— or —NH—, and R¹¹⁴ is a C₁-C₆ straight, branched or cyclic alkylene group, C₃-C₁₀ alkenylene or phenylene group, which may contain a carbonyl, ester, ether or hydroxyl moiety. R¹⁰³, R¹⁰⁴, R¹⁰⁷, R¹⁰⁸, R¹⁰⁹, R¹¹¹, R¹¹², and R¹¹³ are each independently a C₁-C₁₂ straight, branched or cyclic alkyl group which may contain a carbonyl, ester or ether moiety, or a C₆-C₁₂ aryl, C₇-C₂₀ aralkyl, or mercaptophenyl group in which at least one hydrogen (one or more or even all hydrogen atoms) may be replaced by a C₁-C₁₀ straight, branched or cyclic alkyl, halogen, trifluoromethyl, cyano, nitro, hydroxyl, alkoxy, alkoxycarbonyl or acyloxy moiety. R¹⁰⁶ is hydrogen or trifluoromethyl. Z¹ is a single bond or —C(═O)—Z³—R¹¹⁵—. Z² is a single bond, methylene, ethylene, phenylene, fluorinated phenylene, —O—R¹¹⁵— or —C(═O)—Z³—R¹¹⁵—, wherein Z³ is —O— or —NH—, and R¹¹⁵ is a C₁-C₆ straight, branched or cyclic alkylene group, phenylene group, or C₁-C₆ straight, branched or cyclic alkenylene group, which may contain a carbonyl, ester, ether or hydroxyl moiety. A pair of R¹⁰³ and R¹⁰⁴, R¹⁰⁷ and R¹⁰⁸, R¹⁰⁷ and R¹⁰⁹, R¹⁰⁸ and R¹⁰⁹, R¹¹¹ and R¹¹², R¹¹¹ and R¹¹³, or R¹¹² and R¹¹³ may bond directly or via a methylene moiety or ether bond to form a ring with the sulfur atom to which they are attached. Q⁻ is a non-nucleophilic counter ion, c1, c2 and c3 are numbers in the range of 0≤c1≤0.5, 0≤c2≤0.5, 0≤c3≤0.5, and 0<c1+c2+c3≤0.5.

The binding of the acid generator to the polymer backbone is effective for shortening the distance of acid diffusion and reducing LWR.

The inclusion of recurring units (c1), (c2) or (c3) in the base resin is effective for increasing the sensitivity of the resist film by the mechanism that the metal in unit (a) emits secondary electrons during exposure, which induce decomposition of the acid generator in unit (c1), (c2) or (c3). Although a sensitivity increase can be achieved by elevating the PEB temperature or prolonging the PEB time, the acid diffusion distance is increased in either case, resulting in exaggerated LWR. In contrast, the inclusion in the base resin of recurring units containing a rubidium or cesium salt of α-fluorinated sulfonic acid ensures a high sensitivity and low LWR because of suppressed acid diffusion and a high efficiency of acid generation. Of the recurring units (c1), (c2) and (c3), units (c2) are most preferred.

Examples of the monomer from which recurring units (c1) are derived are shown below, but not limited thereto. Q⁻ is as defined above.

Examples of the monomer from which recurring units (c2) are derived are shown below, but not limited thereto.

Examples of the monomer from which recurring units (c3) are derived are shown below, but not limited thereto.

In the base resin, recurring units (d) having a phenolic hydroxyl group as the adhesive group may be further incorporated. Examples of the monomer from which recurring units (d) are derived are shown below, but not limited thereto.

Recurring units (e) having another adhesive group may also be incorporated in the base resin. Examples of the other adhesive group include hydroxyl (other than the phenolic hydroxyl), carboxyl, lactone ring, carbonate, thiocarbonate, carbonyl, cyclic acetal, ether, ester, sulfonic acid ester, cyano, amide, and —O—C(═O)-G- wherein G is sulfur or NH. Examples of the monomer from which recurring units (e) are derived are shown below, but not limited thereto.

In the case of a hydroxyl-containing monomer, a corresponding monomer in which the hydroxyl group has been replaced by an acetal group which is susceptible to deprotection with acid, typically ethoxyethoxy, may be used, and polymerization be followed by deprotection with weak acid and water. Alternatively, the hydroxyl group may have been replaced by an acetyl, formyl or pivaloyl group, and polymerization be followed by alkaline hydrolysis.

The base resin may further comprise recurring units (f) derived from indene, benzofuran, benzothiophene, acenaphthylene, chromone, coumarin, norbornadiene, and derivatives thereof. Examples of the monomer from which recurring units (f) are derived are shown below, but not limited thereto.

The base resin may further comprise recurring units (g) derived from styrene, vinylnaphthalene, vinylanthracene, vinylpyrene, methyleneindane, and derivatives thereof.

In a base resin having recurring units (a), (b1), (b2), (c1), (c2), (c3), (d), (e), (f) and (g) copolymerized therein, the fraction of each unit may preferably fall in the range:

0<a≤0.5, 0≤b1≤0.9, 0≤b2≤0.9, 0<b1+b2<1, 0≤c1≤0.5, 0≤c2≤0.5, 0≤c3≤0.5, 0≤c1+c2+c3≤0.5, 0≤d≤0.9, 0≤e≤0.9, 0≤f≤0.9, and 0≤g≤0.5;

more preferably 0.01≤a≤0.4, 0≤b1≤0.8, 0≤b2≤0.8, 0.1≤b1+b2≤0.8, 0≤c1≤0.4, 0≤c2≤0.4, 0≤c3≤0.4, 0≤c1+c2+c3≤0.4, 0≤d≤0.8, 0≤e≤0.8, 0≤f≤0.8, and 0≤g≤0.4;

even more preferably 0.02≤a≤0.3, 0≤b1≤0.7, 0≤b2≤0.7, 0.15≤b1+b2≤0.7, 0≤c1≤0.3, 0≤c2≤0.3, 0≤c3≤0.3, 0≤c1+c2+c3≤0.3, 0≤d≤0.7, 0≤e≤0.7, 0≤f≤0.7, and 0≤g≤0.3; provided a+b1+b2+c1+c2+c3+d+e+f+g=1.

The base resin may be synthesized by any desired method, for example, by dissolving monomers corresponding to the respective units (a), (b1), (b2), (c1), (C2), (c3), (d), (e), (f) and (g) in an organic solvent, adding a radical polymerization initiator thereto, and effecting heat polymerization. Examples of the organic solvent which can be used for polymerization include toluene, benzene, tetrahydrofuran, diethyl ether and dioxane. Examples of the polymerization initiator used herein include 2,2′-azobisisobutyronitrile (AIBN), 2,2′-azobis(2,4-dimethylvaleronitrile), dimethyl 2,2-azobis(2-methylpropionate), benzoyl peroxide, and lauroyl peroxide. Preferably the system is heated at 50 to 80° C. for polymerization to take place. The reaction time is 2 to 100 hours, preferably 5 to 20 hours.

When hydroxystyrene or hydroxyvinylnaphthalene is to be copolymerized, one possible procedure is by carrying out polymerization using acetoxystyrene or acetoxyvinylnaphthalene instead of hydroxystyrene or hydroxyvinylnaphthalene, and effecting alkaline hydrolysis for deprotection of the acetoxy group for converting back to hydroxystyrene or hydroxyvinylnaphthalene units. Suitable bases used for alkaline hydrolysis include ammonia water and triethylamine. The reaction conditions include a temperature of −20° C. to 100° C., preferably 0° C. to 60° C. and a time of 0.2 to 100 hours, preferably 0.5 to 20 hours.

The base resin should preferably have a weight average molecular weight (Mw) in the range of 1,000 to 500,000, and more preferably 2,000 to 30,000, as measured in tetrahydrofuran solvent by GPC versus polystyrene standards. With a Mw of at least 1,000, the resist composition is fully heat resistant. A polymer with a Mw of up to 500,000 may be devoid of a loss of alkaline solubility or a footing phenomenon after pattern formation.

If a multi-component copolymer has a broad molecular weight distribution or dispersity (Mw/Mn), which indicates the presence of lower and higher molecular weight polymer fractions, there is a possibility that following exposure, foreign matter is left on the pattern or the pattern profile is exacerbated. The influences of molecular weight and dispersity become stronger as the pattern rule becomes finer. Therefore, the base resin should preferably have a narrow dispersity (Mw/Mn) of 1.0 to 2.0, especially 1.0 to 1.5, in order to provide a resist composition suitable for micropatterning to a small feature size.

It is acceptable to use as the base resin a blend of two or more such polymers which differ in compositional ratio, Mw or Mw/Mn or a blend of a polymer containing units (a) and (b) with another polymer containing units (b) to (g), but not unit (a).

The base resin defined above is especially suited as a base resin in a positive resist composition. When a positive resist composition is prepared by using the relevant base resin and combining it with suitable other components such as organic solvent, acid generator, dissolution inhibitor, basic compound, and surfactant, the resist composition has a very high sensitivity in that the polymer in the exposed region accelerates its dissolution rate in developer through catalytic reaction. The resist composition has many advantages including a high dissolution contrast, a high resolution, exposure latitude, process adaptability, a good pattern profile after exposure, high etch resistance, and minimized proximity bias due to controlled acid diffusion. Because of these advantages, the resist composition is fully viable in commercial processes and best suited as the micropatterning resist material for the fabrication of VLSIs.

Specifically, when an acid generator is added to formulate a chemically amplified positive resist composition capable of utilizing acid catalyzed reaction, a higher sensitivity is given and the aforementioned properties are further improved. When a dissolution inhibitor is added to the positive resist composition, the difference in dissolution rate between the exposed and unexposed regions is enhanced, with the resolution being further improved. When a basic compound is added, the rate of acid diffusion in the resist film can be suppressed, with the resolution being further improved. When a surfactant is added, the resist composition is further improved or controlled in coating operation.

In one embodiment, an acid generator is added to the resist composition in order that the composition function as a chemically amplified positive resist composition. Typical of the acid generator used herein is a photoacid generator (PAG) capable of generating an acid in response to actinic radiation or high-energy radiation. Suitable PAGs include sulfonium salts, iodonium salts, sulfonyldiazomethane, N-sulfonyloxyimide, and oxime-O-sulfonate acid generators. Exemplary acid generators are described in U.S. Pat. No. 7,537,880 (JP-A 2008-111103, paragraphs [0122]-[0142]), JP-A 2009-080474, and JP-A 2015-026064. The PAGs may be used alone or in admixture. When used, the acid generator is preferably added in an amount of 0.1 to 50 parts by weight per 100 parts by weight of the base resin.

Examples of the organic solvent used herein are described in JP-A 2008-111103, paragraphs [0144]-[0145]. Illustrative, non-limiting, examples of the organic solvent include ketones such as cyclohexanone and methyl-2-n-pentyl ketone; alcohols such as 3-methoxybutanol, 3-methyl-3-methoxybutanol, 1-methoxy-2-propanol, and 1-ethoxy-2-propanol; ethers such as propylene glycol monomethyl ether, ethylene glycol monomethyl ether, propylene glycol monoethyl ether, ethylene glycol monoethyl ether, propylene glycol dimethyl ether, and diethylene glycol dimethyl ether; esters such as propylene glycol monomethyl ether acetate (PGMEA), propylene glycol monoethyl ether acetate, ethyl lactate, ethyl pyruvate, butyl acetate, methyl 3-methoxypropionate, ethyl 3-ethoxypropionate, t-butyl acetate, t-butyl propionate, and propylene glycol mono-t-butyl ether acetate; and lactones such as γ-butyrolactone. These solvents may be used alone or in combinations of two or more. An appropriate amount of the organic solvent used is 100 to 10,000 parts, especially 200 to 8,000 parts by weight per 100 parts by weight of the base resin.

Exemplary basic compounds or quenchers used herein are described in JP-A 2008-111103, paragraphs [0146]-[0164], and exemplary surfactants in paragraphs [0165]-[0166]. Examples of the dissolution inhibitor are described in JP-A 2008-122932, paragraphs [0155]-[0178] (U.S. Pat. No. 7,771,914). Also polymeric quenchers as described in JP-A 2008-239918 may be added. If necessary, acetylene alcohols may be added, for example, those described in JP-A 2008-122932, paragraphs [0179]-[0182].

Since the polymeric quencher segregates at the surface of a resist film as coated, it is effective for rendering the resist pattern more rectangular. The polymeric quencher is also effective for reducing a film thickness loss and preventing the pattern from being rounded at the top when a protective topcoat for the immersion lithography is applied.

Also a metal salt may be added as the quencher while exemplary metal salts are described in JP-A 2013-025211 (U.S. Pat. No. 9,360,753).

When used, the quencher is preferably added in an amount of 0.01 to 20 parts, more preferably 0.02 to 15 parts by weight per 100 parts by weight of the base resin. When used, the surfactant is preferably added in an amount of 0.0001 to 10 parts, more preferably 0.001 to 5 parts by weight per 100 parts by weight of the base resin. When used, the dissolution inhibitor is preferably added in an amount of 0.5 to 50 parts, more preferably 1 to 30 parts by weight per 100 parts by weight of the base resin. When used, the polymeric quencher may be added in any desired amount as long as the benefits of the invention are not impaired. When used, the acetylene alcohol is preferably added in an amount of 0 to 2%, more preferably 0.02 to 1% by weight based on the resist composition.

Process

Another embodiment of the invention is a pattern forming process comprising the steps of coating the resist composition defined above onto a substrate, baking the coating to form a resist film, exposing the resist film to high-energy radiation, and developing the exposed resist film with a developer. The step of exposing the resist film to high-energy radiation may use EUV of wavelength 3 to 15 nm or EB, specifically EB at an accelerating voltage of 1 to 150 kV.

Since the rubidium or cesium salt has a strong absorption of EUV of wavelength 13.5 nm, the sensitivity of the resist film is improved upon exposure to EUV by the mechanism that the outer shell electrons of the metal are excited, and the electrons transfer to the acid generator, whereby the efficiency of acid generation is enhanced.

Since the rubidium or cesium salt of α-fluorinated sulfonic acid does not undergo salt exchange with fluorosulfonic acid or fluorosulfonimidic acid generated from the acid generator, it does not function as a quencher. Thus the advantages of increased absorption of EUV and increased sensitivity are available.

When the resist composition is used for the microfabrication of various integrated circuits, any well-known lithography processes may be applied. Upon imagewise exposure to EB or EUV, the rubidium or cesium salt generates secondary electrons, to which the acid generator is sensitive. This leads to a high sensitivity. However, if secondary electrons randomly diffuse in the resist film, the image is blurred. With this combined with diffusion of the acid generated by the acid generator, the image blur is exaggerated, inviting an increase of edge roughness. If secondary electrons diffuse in the thickness direction of the resist film, i.e., perpendicular to the substrate, then the image blur is suppressed. When the substrate is electrically charged positive (+), secondary electrons move as if they were sucked into the substrate, that is, secondary electrons diffuse perpendicularly. Then the sensitivity can be improved while suppressing the image blur, and without degrading the edge roughness.

For example, the resist composition is applied onto a substrate for integrated circuit fabrication or a processable layer thereon (e.g., Si, SiO₂, SiN, SiON, TiN, WSi, BPSG, SOG, or organic antireflective coating) or a substrate for mask circuit fabrication or a processable layer thereon (e.g., Cr, CrO, CrON, MoSi, or SiO₂) by any suitable technique such as spin coating, roll coating, flow coating, dip coating, spray coating or doctor coating. The coating is prebaked on a hot plate at a temperature of 60 to 150° C. for 10 seconds to 30 minutes, preferably 80 to 120° C. for 30 seconds to 20 minutes to form a resist film having a thickness of 0.1 to 2.0 μm.

Next the resist film is exposed imagewise to high-energy radiation selected from among UV, DUV, EB, x-ray, excimer laser, γ-ray, synchrotron radiation, or EUV directly or through a mask having the desired pattern. The exposure is preferably carried out to provide a dose of 1 to 200 mJ/cm², preferably 10 to 100 mJ/cm², or 0.1 to 100 μC/cm², preferably 0.5 to 50 μC/cm². This is followed by baking (PEB) on a hot plate at 60 to 150° C. for 10 seconds to 30 minutes, preferably at 80 to 120° C. for 30 seconds to 20 minutes.

Finally, the exposed resist film is developed with a developer which is an aqueous alkaline solution, typically a 0.1 to 5%, preferably 2 to 3% by weight of tetramethylammonium hydroxide (TMAH), choline hydroxide, tetraethylammonium hydroxide (TEAH), tetrapropylammonium hydroxide (TPAH), tetrabutylammonium hydroxide (TBAH), benzyltrimethylammonium hydroxide or benzyltriethylammonium hydroxide. Development is carried out for 3 seconds to 3 minutes, preferably 5 seconds to 2 minutes by any conventional techniques such as dip, puddle and spray techniques. The exposed region of resist film is dissolved in the developer, but not the unexposed region. In this way, the desired positive pattern is formed on the substrate.

Alternatively, a negative tone pattern may be formed by organic solvent development. The organic solvent used as the developer is preferably selected from 2-octanone, 2-nonanone, 2-heptanone, 3-heptanone, 4-heptanone, 2-hexanone, 3-hexanone, diisobutyl ketone, methylcyclohexanone, acetophenone, methylacetophenone, propyl acetate, butyl acetate, isobutyl acetate, pentyl acetate, isopentyl acetate, butenyl acetate, propyl formate, butyl formate, isobutyl formate, pentyl formate, isopentyl formate, methyl valerate, methyl pentenoate, methyl crotonate, ethyl crotonate, methyl propionate, ethyl propionate, ethyl 3-ethoxypropionate, methyl lactate, ethyl lactate, propyl lactate, butyl lactate, isobutyl lactate, pentyl lactate, isopentyl lactate, methyl 2-hydroxyisobutyrate, ethyl 2-hydroxyisobutyrate, methyl benzoate, ethyl benzoate, phenyl acetate, benzyl acetate, methyl phenylacetate, benzyl formate, phenylethyl formate, methyl 3-phenylpropionate, benzyl propionate, ethyl phenylacetate, and 2-phenylethyl acetate. These organic solvents may be used alone or in admixture of two or more.

At the end of development, the resist film is rinsed. As the rinsing liquid, a solvent which is miscible with the developer and does not dissolve the resist film is preferred. Suitable solvents include alcohols of 3 to 10 carbon atoms, ether compounds of 8 to 12 carbon atoms, alkanes, alkenes, and alkynes of 6 to 12 carbon atoms, and aromatic solvents. Specifically, suitable alcohols of 3 to 10 carbon atoms include n-propyl alcohol, isopropyl alcohol, 1-butyl alcohol, 2-butyl alcohol, isobutyl alcohol, t-butyl alcohol, 1-pentanol, 2-pentanol, 3-pentanol, t-pentyl alcohol, neopentyl alcohol, 2-methyl-1-butanol, 3-methyl-1-butanol, 3-methyl-3-pentanol, cyclopentanol, 1-hexanol, 2-hexanol, 3-hexanol, 2,3-dimethyl-2-butanol, 3,3-dimethyl-1-butanol, 3,3-dimethyl-2-butanol, 2-ethyl-1-butanol, 2-methyl-1-pentanol, 2-methyl-2-pentanol, 2-methyl-3-pentanol, 3-methyl-1-pentanol, 3-methyl-2-pentanol, 3-methyl-3-pentanol, 4-methyl-1-pentanol, 4-methyl-2-pentanol, 4-methyl-3-pentanol, cyclohexanol, and 1-octanol. Suitable ether compounds of 8 to 12 carbon atoms include di-n-butyl ether, diisobutyl ether, di-s-butyl ether, di-n-pentyl ether, diisopentyl ether, di-s-pentyl ether, di-t-pentyl ether, and di-n-hexyl ether. Suitable alkanes of 6 to 12 carbon atoms include hexane, heptane, octane, nonane, decane, undecane, dodecane, methylcyclopentane, dimethylcyclopentane, cyclohexane, methylcyclohexane, dimethylcyclohexane, cycloheptane, cyclooctane, and cyclononane. Suitable alkenes of 6 to 12 carbon atoms include hexene, heptene, octene, cyclohexene, methylcyclohexene, dimethylcyclohexene, cycloheptene, and cyclooctene. Suitable alkynes of 6 to 12 carbon atoms include hexyne, heptyne, and octyne. Besides the foregoing solvents, aromatic solvents may be used, for example, toluene, xylene, ethylbenzene, isopropylbenzene, t-butylbenzene and mesitylene. The solvents may be used alone or in admixture.

Of the variety of high-energy radiation, the resist composition is best suited in micropatterning with EB, EUV, x-ray, soft x-ray, γ-ray, or synchrotron radiation. Particularly when EUV of wavelength 3 to 15 nm or an accelerated EB at an accelerating voltage of 1 to 150 kV, preferably up to 100 kV, especially a low voltage accelerated EB at an accelerating voltage of up to 50 kV is used, a finer size pattern can be formed.

EXAMPLE

Examples are given below by way of illustration of the invention and not by way of limitation. The abbreviation “pbw” is parts by weight. For all polymers, Mw and Mn are determined versus polystyrene standards by GPC using tetrahydrofuran (THF) solvent.

Synthesis Example Synthesis Example 1

Synthesis of Metal Salt Monomers

Metal salt monomers were synthesized by mixing a benzyltrimethylammonium salt of an α-fluorinated sulfonic acid having a polymerizable double bond with cesium hydroxide or rubidium hydroxide, and washing the mixture with water to remove benzyltrimethylammonium hydroxide.

Synthesis Example 2

Synthesis of Base Resins

Various base resins (Polymers 1 to 5, Comparative Polymer 1) were prepared by combining suitable monomers, effecting copolymerization reaction in THF solvent, pouring into methanol for crystallization, repeatedly washing with hexane, isolation, and drying. The base resins were analyzed by ¹H-NMR to determine their composition and by GPC to determine Mw and dispersity Mw/Mn.

Examples 1 to 7 & Comparative Example 1

Preparation of Resist Composition

Positive resist compositions in solution form were prepared by dissolving a base resin (synthesized above) and selected components in a solvent in accordance with the formulation of Table 1 and filtering through a filter with a pore size of 0.2 μm. The solvent contained 100 ppm of surfactant FC-4430 (commercially available from 3M). The components in Table 1 are identified below.

-   Acid generator: PAG1 of the following structural formula

-   Basic compound or quencher: Amine 1, Amine 2, and cesium pivalate of     the following structural formulae

Organic Solvents:

PGMEA (propylene glycol monomethyl ether acetate)

CyH (cyclohexanone)

GBL (γ-butyrolactone)

TABLE 1 Polymer Acid generator Quencher Organic solvent Resist (pbw) (pbw) (pbw) (pbw) Example 1 Resist 1 Polymer 1 — Amine 1 PGMEA(1,000) (100) (1.3) CyH(1,000) GBL(200) 2 Resist 2 Polymer 2 — Amine 1 PGMEA(1,000) (100) (1.3) CyH(1,000) GBL(200) 3 Resist 3 Polymer 3 — Amine 1 PGMEA(1,000) (100) (1.3) CyH(1,000) GBL(200) 4 Resist 4 Polymer 4 — Amine 1 PGMEA(1,000) (100) (1.3) CyH(1,000) GBL(200) 5 Resist 5 Polymer 5 — Amine 1 PGMEA(1,000) (100) (1.3) CyH(1,000) GBL(200) 6 Resist 6 Polymer 1 PAG1 Amine 2 PGMEA(1,000) (100) (10) (0.8) CyH(1,000) GBL(200) 7 Resist 7 Polymer 1 — cesium PGMEA(1,000) (100) pivalate CyH(1,000) (1.2) GBL(200) Comparative 1 Comparative Comparative — Amine 1 PGMEA(1,000) Example Resist 1 Polymer 1 (1.3) CyH(1,000) (100) GBL(200) EB Writing Test

Using a coater/developer system Clean Track Mark 5 (Tokyo Electron Ltd.), the positive resist composition was spin coated onto a silicon substrate (diameter 6 inches, vapor primed with hexamethyldisilazane (HMDS)) and pre-baked on a hot plate at 110° C. for 60 seconds to form a resist film of 70 nm thick. Using a system HL-800D (Hitachi Ltd.) at a HV voltage of 50 kV, the resist film was exposed imagewise to EB in a vacuum chamber.

Using Clean Track Mark 5, immediately after the imagewise exposure, the resist film was baked (PEB) on a hot plate at the temperature shown in Table 2 for 60 seconds and puddle developed in a 2.38 wt % tetramethylammonium hydroxide (TMAH) aqueous solution for 30 seconds to form a positive pattern.

Sensitivity is the exposure dose that provides a 1:1 resolution of a 100-nm line-and-space pattern. The 100-nm line-and-space pattern at that dose was measured for line width roughness (LWR) under SEM.

The resist composition is shown in Table 2 along with the sensitivity and LWR of EB lithography.

TABLE 2 PEB temp. Sensitivity LWR Resist (° C.) (μC/cm²) (nm) Example 1 Resist 1 90 24.5 5.9 2 Resist 2 90 25.5 5.2 3 Resist 3 90 23.5 5.6 4 Resist 4 90 26.0 5.2 5 Resist 5 80 29.1 5.0 6 Resist 6 90 27.0 5.3 7 Resist 7 90 21.0 5.1 Comparative 1 Comparative 90 32.5 7.2 Example Resist 1

It is evident from Table 2 that the resist compositions of Examples have a high sensitivity and reduced LWR. The resist composition of Comparative Example has sensitivity and LWR values which are inferior to those of Examples. It is demonstrated that the resist composition comprising a polymer comprising acid labile group-containing recurring units and recurring units containing a rubidium or cesium salt of α-fluorinated sulfonic acid and preferably acid generator-bearing recurring units exhibits a high resolution, a high sensitivity, and a minimal LWR. The resist composition is best suited as the resist material for VLSIs and patterning material for masks.

It is noted that the invention is not limited to the aforementioned embodiments. While the embodiments are merely exemplary, any embodiments having substantially the same construction as the technical concept set forth in the following claims and exerting equivalent functions and results are believed to be within the spirit and scope of the invention.

Japanese Patent Application No. 2015-208575 is incorporated herein by reference.

Although some preferred embodiments have been described, many modifications and variations may be made thereto in light of the above teachings. It is therefore to be understood that the invention may be practiced otherwise than as specifically described without departing from the scope of the appended claims. 

The invention claimed is:
 1. A resist composition comprising a base resin comprising recurring units containing an acid labile group and recurring units having the formula (a):

wherein R¹ is hydrogen or methyl, X¹ is phenylene or —C(═O)—X³—R²—, X³ is —O— or —NH—, R² is a single bond, or a C₁-C₂₀ straight, branched or cyclic alkylene group, C₂-C₂₀ straight, branched or cyclic alkenylene group, C₂-C₂₀ straight, branched, or cyclic alkynylene group, or C₆-C₁₀ aryl group, which may contain an ether, ester, lactone ring, carbonate, amide, carbamate, sulfonic acid ester or sultone moiety, X² is a single bond, a C₁-C₂₀ straight, branched or cyclic alkyl group, C₂-C₂₀ straight, branched or cyclic alkenyl group, C₂-C₂₀ straight, branched or cyclic alkynyl group, or C₆-C₁₀ aryl group, which may contain halogen, ether, thiol, ester, carbonate, carbonyl, amide, amino, azide, carbamate, nitro, cyano, hydroxyl, carboxyl, sulfo, sulfonic acid ester, sultone, lactone ring or lactam ring, and M⁺ is a rubidium or cesium ion.
 2. The resist composition of claim 1 wherein the recurring units containing an acid labile group have the formula (b1) or (b2):

wherein R³ and R⁵ are each independently hydrogen or methyl, R⁴ and R⁶ each are an acid labile group, Y¹ is a single bond, a C₁-C₁₂ linking group having at least one of ester, lactone ring, phenylene and naphthylene moiety, a phenylene group, or a naphthylene group, Y² is a single bond, ester group or amide group, b1 and b2 are numbers in the range: 0≤b1≤0.9, 0≤b2≤0.9, and 0<b1+b2<1.
 3. The resist composition of claim 2 wherein the base resin further comprises recurring units of at least one type selected from the formulae (c1), (c2) and (c3):

wherein R¹⁰¹, R¹⁰⁵, and R¹¹⁰ each are hydrogen or methyl, R¹⁰² is a single bond, phenylene, —O—R¹¹⁴—, or —C(═O)—Y—R¹¹⁴—, Y is —O— or —NH—, R¹¹⁴ is a C₁-C₆ straight, branched or cyclic alkylene group, C₃-C₁₀ alkenylene or phenylene group, which may contain a carbonyl, ester, ether or hydroxyl moiety, R¹⁰³, R¹⁰⁴, R¹⁰⁷, R¹⁰⁸, R¹⁰⁹, R¹¹¹, R¹¹², and R¹¹³ are each independently a C₁-C₁₂ straight, branched or cyclic alkyl group which may contain a carbonyl, ester or ether moiety, or a C₆-C₁₂ aryl, C₇-C₂₀ aralkyl, or mercaptophenyl group in which at least one hydrogen may be replaced by a C₁-C₁₀ straight, branched or cyclic alkyl, halogen, trifluoromethyl, cyano, nitro, hydroxyl, alkoxy, alkoxycarbonyl or acyloxy moiety, R¹⁰⁶ is hydrogen or trifluoromethyl, Z¹ is a single bond or —C(═O)—Z³—R¹¹⁵—, Z² is a single bond, methylene, ethylene, phenylene, fluorinated phenylene, —O—R¹¹⁵— or —C(═O)—Z³—R¹¹⁵—, Z³ is —O— or —NH—, R¹¹⁵ is a C₁-C₆ straight, branched or cyclic alkylene group, phenylene group, or C₁-C₆ straight, branched or cyclic alkenylene group, which may contain a carbonyl, ester, ether or hydroxyl moiety, a pair of R¹⁰³ and R¹⁰⁴, R¹⁰⁷ and R¹⁰⁸, R¹⁰⁷ and R¹⁰⁹, R¹⁰⁸ and R¹⁰⁹, R¹¹¹ and R¹¹², R¹¹¹ and R¹¹³, or R¹¹² and R¹¹³ may bond directly or via a methylene moiety or ether bond to form a ring with the sulfur atom to which they are attached, Q⁻ is a non-nucleophilic counter ion, c1, c2 and c3 are numbers in the range of 0≤c1≤0.5, 0≤c2≤0.5, 0≤c3≤0.5, and 0<c1+c2+c3≤0.5.
 4. The resist composition of claim 3 wherein the base resin comprises recurring units having formula (c2).
 5. The resist composition of claim 1 which is a chemically amplified positive resist composition.
 6. The resist composition of claim 1, further comprising at least one component selected from among an organic solvent, a dissolution inhibitor, an acid generator, a basic compound, and a surfactant.
 7. A pattern forming process comprising the steps of coating the resist composition of claim 1 onto a substrate, baking, exposing the resulting resist film to high-energy radiation, and developing with a developer.
 8. The process of claim 7 wherein the high-energy radiation is EUV radiation having a wavelength of 3 to 15 nm.
 9. The process of claim 7 wherein the high-energy radiation is an electron beam at an accelerating voltage of 1 to 150 kV.
 10. The process of claim 8 wherein in the step of exposing the resist film to high-energy radiation, the surface of the substrate underneath the resist film is electrically charged positive.
 11. The resist composition of claim 1 wherein the recurring unit having the formula (a) is derived from a monomer selected from the group consisting of the following formulae:


12. The resist composition of claim 1 wherein M⁺ is a rubidium ion. 